Exchange of matter at the interface between two liquid phases

Theoretical models of the exchange of matter at interfaces are necessary to interpret relaxation phenomena in surfactant adsorption layers. For liquid-liquid systems, the diffusion-controlled exchanges of surfactant molecules at the interfaces can be influenced by a simultaneous transfer of molecules from one bulk phase to the other. The diffusional mass exchange function is derived taking into account the transfer across the interface. The resulting mass exchange function is used to calculate the interfacial tension response of a liquid-liquid system. As an example, the interfacial response after a ramp-type area perturbation is calculated.     

Ab-initio self-consistent field theory for the treatment of interface between two different quasi-one-dimensional chains

The solution of the problem how to treat an interface between two quasi-one-dimensional polymers has been formulated combining Dyson's equation with the mutually consistent field (self-consistent field solutions for the change of the bulk states of the polymers and for the extraband “interface” states) method. A numerical procedure for the implementation of an appropriate program (which needs as input only the ab initio self-consistent field crystal orbital solutions of the two infinite periodic polymers as well as some cluster calculations of the interface and its neighbors at both sides) is outlined. The necessary program is under development.     

Polymer lateral diffusion at the solid-liquid interface

Measurements are presented of how polymer surface diffusion at the solid-liquid interface is controlled by surface coverage. The method of measurement was fluorescence correlation spectroscopy (FCS), and the system was poly(ethylene oxide) (PEG) adsorbed onto methyl-terminated self-assembled monolayers in buffered aqueous solution. The translational diffusion coefficient at first increased with increasing surface concentration, presumably because the number of adsorption sites per molecule decreased. Ultimately it slowed by 1 order of magnitude, presumably reflecting jamming by neighboring chains.     

Solid state photoreactions occurring at the interface between crystallites of two different organic compounds

This paper deals with the solid state bimolecular photoreactions occurring at the interface between crystallites of two different organic compounds prepared as a polycrystalline mixture by melting-resolidification. The photoreactions include the photoaddition of NH compounds to aromatic and olefinic double bonds, hydrogen abstraction, condensation reaction via photoelectron transfer and [2+2] photoaddition. Particular attention is focused on the characterization of the polycrystalline mixture, the selectivity of the photoreactions in the solid state compared with those in the solution phase and the factors controlling the photoreactions.     

Polymer/surfactant interactions at the air/water interface

The development of neutron reflectometry has transformed the study and understanding of polymer/surfactant mixtures at the air/water interface. A critical assessment of the results from this technique is made by comparing them with the information available from other techniques used to investigate adsorption at this interface. In the last few years, detailed information about the structure and composition of adsorbed layers has been obtained for a wide range of polymer/surfactant mixtures, including neutral polymers and synthetic and naturally occurring polyelectrolytes, with single surfactants or mixtures of surfactants. The use of neutron reflectometry together with surface tensiometry, has allowed the surface behaviour of these mixtures to be related directly to the bulk phase behaviour. We review the broad range of systems that have been studied, from neutral polymers with ionic surfactants to oppositely charged polyelectrolyte/ionic surfactant mixtures. A particular emphasis is placed upon the rich pattern of adsorption behaviour that is seen in oppositely charged polyelectrolyte/surfactant mixtures, much of which had not been reported previously. The strong surface interactions resulting from the electrostatic attractions in these systems have a very pronounced effect on both the surface tension behaviour and on adsorbed layers consisting of polymer/surfactant complexes, often giving rise to significant surface ordering.     

Specifics of localization of carbon black at the interface between polymeric phases

The thermodynamic and kinetic factors that determine the localization of carbon black particles at interfaces in polymer blends were established. Substantial differences in particle localization conditions at the interfaces of two high-molecular-mass or low-molecular-mass liquids were revealed; these differences are manifested in the determining effect of the sequence of mixing of the filler with the polymeric components of the blend on the localization. The effect is explained in terms of nonequilibrium positions of carbon black particles at the interface. This nonequilibrium distribution is primarily due to a high energy of desorption of macromolecules from the solid surface.     

A computer simulation study of water drying at the interface of protein chains

This study investigated the water drying (cavitation) in the interfacial region of two chains of a dimeric protein by nanosecond molecular dynamics simulations using explicit water representation. Separation-induced cavity of water was directly observed in the region. We evaluated the separation length scale of two chains on which the drying transition occurs, and the average number of water molecules that are expelled from the interfacial region during the transition. The obtained values can be rationalized by Kelvin equation for finite lateral size of confinement [K. Lum and A. Luzar, Phys. Rev. E 56, R6283 (1997)]. Also, we found that the drying transition is accompanied by an exponential reduction in the average hydrogen-bond number per interfacial water molecule. The results of this study may deepen the understanding of how hydrophobic interaction drives the assembly of protein chains.     

Stability of the interface between two dynamic phases in capillary flow of linear polymer melts

Results of previous work¹ on a theoretical explanation of the “spurt effect” in polymer melt flow are extended. A modified Doi—Edwards liquid is shown to support axisymmetric traveling waves on an interface between high and low shear-rate phases in capillary flow. The stability of these perturbations is found to be governed by normal stress effects and may be related to certain types of melt fracture. Observed effects of varying the capillary length are explained qualitatively.     

Optodynamers: expression of color and fluorescence at the interface between two films of different dynamic polymers

Color and fluorescence changes were expressed at the interface between two different polyhydrazone polymer films, induced by hydrazone bond exchange and component recombination through the interface, resulting in an extension of conjugation.     

Polymer chain dynamics at interfaces: role of boundary conditions at solid interface

Using classical molecular dynamics simulations, we study the dynamical properties of a single polymer chain dissolved in an explicit solvent and strongly adsorbed at solid-liquid interface. To circumvent a serious challenge posed by finite size effects due to long-range hydrodynamic effects, we developed a correction procedure that substantially limits the finite size effects. Concurrently, we provide an analysis of distinctly different size effects in the directions transverse and normal to the interface. We find that on analytically smooth interfaces, corresponding to the slip boundary condition, the motions of the polymer chain and the surrounding solvent are hydrodynamically coupled. This leads to the chain diffusion coefficient D scaling with the chain degree of polymerization N as D approximately N(-3/4), consistent with the Zimm dynamics for strongly adsorbed chains. Introduction of transverse forces at the interface results in loss of correlation between the motion of the polymer chain and the solvent. Consequently, D approximately N(-1), which is a characteristic of the Rouse dynamics.     

Fatty-acid monolayers at the nematic/water interface: phases and liquid-crystal alignment

The two-dimensional (2D) phases of fatty-acid monolayers (hexadecanoic, octadecanoic, eicosanoic, and docosanoic acids) have been studied at the interface of a nematic liquid crystal (LC) and water. When observed between crossed polarizers, the LC responds to monolayer structure owing to mesoscopic alignment of the LC by the adsorbed molecules. Similar to Langmuir monolayers at the air/water interface, the adsorbed monolayer at the nematic/water interface displays distinct thermodynamic phases. Observed are a 2D gas, isotropic liquid, and two condensed mesophases, each with a characteristic anchoring of the LC zenithal tilt and azimuth. By varying the monolayer temperature and surface concentration we observe reversible first-order phase transitions from vapor to liquid and from liquid to condensed. A temperature-dependent transition between two condensed phases appears to be a reversible swiveling transition in the tilt azimuth of the monolayer. Similar to monolayers at the air/water interface, the temperature of the gas/liquid/condensed triple-point temperature increased by about 10 degrees C for a two methylene group increase in chain length. However, the absolute value of the triple-point temperatures are depressed by about 40 degrees C compared to those of analogous monolayers at the air/water interface. We also observe a direct influence by the LC layer on the mesoscopic and macroscopic structure of the monolayer by analyzing the shapes and internal textures of gas domains in coexistence with a 2D liquid. An effective anisotropic line tension arises from elastic forces owing to deformation of the nematic director across phase boundaries. This results in the deformation of the domain from circular to elongated, with a distinct singularity. The LC elastic energy also gives rise to transition zones displaying mesoscopic realignment of the director tilt or azimuth between adjacent regions with a sudden change in anchoring.     

Monolayers of stearic acid esters at the air-water interface: two-dimensional phases and miscibility

Monolayers of three stearic acid esters, methyl-stearate, propyl-stearate and butyl-stearate, were studied at the air-water interface in the 15–35 °C temperature range.To investigate the surface phases of these esters spread at the air-water interface, some state equations were fitted with the experimental data taken from the isotherms.Surface potential measurements were carried out to obtain information on the molecular orientation. The interfacial orientation and distribution was discussed in relationship to the surface phases present.Ellipsometric measurements were made to determine the thicknesses.The two-dimensional miscibility for the mixture methyl-stearate/butyl-stearate was also studied: surface free energies, enthalpies, and entropies of mixing were computed. The results obtained confirmed previous deductions about the role of the hydrophobic chains in determining the two-dimensional miscibility when they have the same interfacial orientation.     

Monolayers of 1,2,3-propanetriol esters at the air-water interface: two-dimensional phases and miscibility

Monolayers of two glycerol esters, 1-monooleoylglycerol and 1-monostearoylglycerol, were studied at the air-water interface in the 15–30C temperature range.From the experimental isotherms (surface pressure vs area and surface potential vs area) the surface phases of the esters were inferred.The miscibility between the two esters was discussed in relationship to the interfacial orientations and distributions and to the surface phases of the esters.The results obtained supported and strengthened the empirical rule on miscibility previously found [1–7], i.e., for non-ionic compounds the same interfacial orientation of the hydrophobic chains is required in order to have bidimensional miscibility between the components.     

Asymmetric banana-shaped liquid crystals with two different terminal alkoxy chains

Two series of asymmetric banana-shaped compounds have been synthesized and studied. In the 1,3-phenylene bis[4-(4'-alkoxybenzoyloxy)]benzoate series the lack of symmetry was derived solely from the difference in length of the two terminal alkoxy chains. In the 3,4'-biphenylene bis[4-(4'-alkoxybenzoyloxy)]benzoate series the asymmetric nature originates from the 3,4'-substitution of the central biphenyl group and from the difference in length of the two terminal chains. All the melting points of the asymmetrical compounds in the series with the central phenyl unit are lower than those of the symmetrical compounds. The liquid crystalline B₁ or B₂ phase was retained in all cases. In the series with the central biphenyl unit the compounds with the shortest chain attached to the para-position of the central biphenyl unit have the lowest melting points. A significant lowering of the melting points in comparison with the symmetrically substituted compounds, however, could not be achieved. All the compounds of both series show a layer spacing which is comparable to those of the symmetrically substituted parent compounds. The observed switching behaviour of both the symmetric and asymmetric compounds with a B₂ phase was antiferroelectric.     

两种带有不同间隔臂的环糊精键合固定相保留行为的评价

选取14种模型化合物对两种带有不同间隔臂的环糊精键合固定相(Click Alkyl-CD、Click OEG-CD)进行了反相液相色谱模式下的保留行为评价。通过梯度洗脱条件下保留参数计算方法和CSASS软件,根据3次线性梯度的保留值数据,测出14种溶质分子在两种固定相上的保留参数,在此基础上考察流动相含乙腈浓度与保留因子的关系后发现,Click Alkyl-CD和Click OEG-CD在分离非极性和中等极性化合物时主要基于反相液相色谱模式,而某些化合物(如吲唑)在Click OEG-CD上的保留受多种作用力影响,并不基于反相液相色谱模式。疏水性评价结果表明,反相分离模式下Click Alkyl-CD的保留参数和正辛醇-水分配常数的相关性较好(R=0.7),说明其具有比较强的疏水性;而Click OEG-CD的相关性不高(R＜0.3),说明疏水作用力以外的其他作用力对化合物在反相模式下的保留影响较大。     

Semifluorinated chains in 2D-(perfluorododecyl)-alkanes at the air/water interface

Langmuir monolayers of a homologous series of perfluorododecyl-n-alkanes (general formula F(CF2)12(CH2)nH, abbreviated as F12Hn, where n = 6-20) are investigated through isotherms of surface pressure (pi) and electric surface potential (DeltaV) versus area (A) and quantitative Brewster angle microscopy. The investigated monolayers are found to be liquid in nature. The negative sign of the measured surface potential evidences the orientation of all the investigated molecules with their perfluorinated parts directed toward the air regardless of the length of the hydrogenated unit. Analysis of the direction of the molecular dipole moment with respect to the main axis indicates that the minimum effective dipole moment is achieved for a molecule oriented at an angle of about 35 degrees to the surface normal. The film thickness was evaluated from the relative intensity measurements. The results suggest that the F12Hn molecules are tilted to the interface in the vicinity of collapse, which is in accordance with the liquid character of their monolayers.     

Beyond the born approximation. The case of very long polymer chains adsorbed at an interface

Two experimental evidences are discussed of the reflectance discontinuity associated with very long adsorbed polymer chains. The anomalous low reflectivity is compared to the Ramsauer-Townsend effect in the scattering of slow electrons by rare-gas atoms.